CO Oxidation on Copper Oxide Cluster Ions

We investigated the reaction of CO and O$_2$ on size-selected copper cluster ions, Cu$_n^+$ ($n$=4--18) and Cu$_n^-$ ($n$=4--11), by use of a tandem mass-spectrometer equipped with octopole ion guides. A coadsorbing product, Cu$_n$O$_2$(CO)$^+$, was observed in the reaction of Cu$_n$O$_2^+$ with CO, and it was found that CO adsorbs onto Cu$_n$O$_2^+$ more efficiently than onto Cu$_n^+$ in $n \geq 9$. This shows the cooperative coadsorption of O$_2$ and CO. On the other hand, in the reaction of Cu$_n$O$_2^-$ with CO, a reduced product, Cu$_n$O$^-$, was obtained instead of the coadsorbing product, Cu$_n$O$_2$(CO)$^-$. In particular, Cu$_5$O$_2^-$ and Cu$_9$O$_2^-$ have relatively high efficiency for the production of Cu$_n$O$^-$. This result suggests the production of CO$_2$ by the oxidation of CO on Cu$_n$O$_2^-$. The DFT calculation indicates that the activation energy in the reaction of Cu$_5$O$_2$(CO)$^-$ $\rightarrow$ Cu$_5$O(CO$_2$)$^-$ is only 0.79 eV while that of the corresponding cation is 1.79 eV. The structure of Cu$_n^-$ is more flexible than that of Cu$_n^+$ probably because of its excess electron. It is concluded that the stabilization of the transition state and the decrease of the activation energy make the CO oxidation proceed on Cu$_n$O$_2^-$.